Predicting who develops posttraumatic stress disorder has remained elusive, as acute neurobiological mechanisms of posttraumatic stress pathology are not well understood. The endocannabinoid signaling system (ECSS) is: involved in the stress response, connected with the limbic and neuroendocrine systems, and has been linked with behavioral responses consistent with anxiety in preclinical studies. The ECSS consists of the cannabinoid receptor (CB1R) and two endogenous ligands (endocannabinoids, eCBs), N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG). For those with non-remitting posttraumatic stress disorder (PTSD), there is evidence of endocannabinoid signaling dysregulation. Less well understood is the ECSS role in risk for PTSD in the acute period after trauma. The objective of this proposal is to characterize ECSS functioning early after trauma and to investigate its link to stress regulation circuitry, HPA axis functioning, affective memory formation and risk for non- remitting PTSD. The specific aims of the proposal are to: 1) Determine trajectories of circulating eCBs following trauma and their relationships with cortisol, arousal symptoms, neural circuits supporting and regulating arousal, and PTSD outcome; 2) Characterize challenge-elicited 2-AG & AEA response as a predictor of PTSD outcome; and 3) Explore the link between AEA with fear memory formation, neural activity in the hippocampal affective memory circuit, and PTSD. This study proposes a longitudinal prospective cohort study of traumatic injury survivors (ie, motor vehicle crash, gunshot and stab wound) evaluated at a level 1 trauma center. Participants will receive a risk screen for PTSD and if risk positive, an initial blood draw in the hospital (2-AG, AEA, cortisol). Acute blood draws and symptom assessments will occur at 1 and 2 weeks, amygdala and medial prefrontal cortex (mPFC) functioning utilizing functional magnetic resonance imaging (fMRI) and structured clinical interview to assess PTSD, depression, and other psychological disorders, along with a challenge task to evaluate eCB responding will occur at 1 month, with additional blood draws and structured clinical interviews occurring at 3, 6, and 12 months. Standardized laboratory techniques will be used to assess the serum for 2-AG, AEA, and cortisol. Statistical analysis will evaluate trajectories of eCBs, their relationships with cortisol, fear and affective memory circuitry, and collectively the capacity of these measurements to predict non-remitting PTSD.